The stator of an AC motor comprises a laminated core mounted with coils, and heat is generated in the stator due to iron loss and copper loss, which has a serious adverse influence on the useful life and performance of the motor. To obviate this adverse influence of the heat generated in the stator on the useful life and performance of the AC motor, methods of removing heat from the stator by circulating cooling air through the laminated core (for example, U.S. Pat. No. 4,534,686) and methods of removing heat from the stator by circulating a cooling liquid capable of efficient heat removal have been provided or proposed.
A liquid-cooled motor in which a cooling liquid is circulated through the laminated core of the stator has an advantage over an air-cooled motor in that the cooling medium is capable of a high heat removing efficiency, and thus the cooling medium cools the motor at high efficiency. Nevertheless, the liquid-cooled motor has a problem in that thermal distortion of the liquid-cooled motor occurs when the laminated core is subjected to the cooling effect of the cooling liquid during the operation of the liquid-cooled motor if the heat distribution in the laminated core and the peripheral parts is not uniform, and accordingly, the rotor will fail to rotate smoothly, and because of the resulting unsteady rotation, the rotor shaft is unable to apply a fixed torque to the driven object associated with the liquid-cooled motor.